1. Field of the Invention
The present invention relates to an autofocus system to perform focus control of a photographic lens, and, more particularly, the present invention relates to an autofocus system including a detachable photographic lens having a drive mechanism for focus control and including a camera body having a focus detection unit.
2. Description of the Related Art
An autofocus system is known, for example, as disclosed in Japanese Laid-Open Patent Publication JP-A-63-304223, which simultaneously carries out in parallel an operation of detecting a defocus amount (the displacement amount of a photographic plane and an image plane) and the drive operation of a lens. In the known autofocus system, the detected value of the defocus amount has to be corrected by the extent of the lens movement after the focus detection. The correction of the detected defocus amount is referred to as "autofocus correction" hereinbelow.
Conventionally, the autofocus correction is performed on the camera body side by the following operations (1)-(3).
(1) Firstly, a defocus amount DF1 determined by a focus detection unit is converted into a drive amount Z1 of the focusing lens within the photographic lens. PA0 (2) Secondly, from the drive amount Z1 of the focusing lens, the actual residual drive amount Z3 is calculated (referred to hereinbelow as "residual drive amount") to reduce the lens movement after focus detection Z2. PA0 (3) Finally, the residual drive amount Z3 is converted into a residual defocus amount DF3 in proportion to a conversion coefficient A (i.e., a ratio of drive amount to defocus amount). PA0 (1) Firstly, the first conversion device converts the defocus amount into the drive amount of the focusing lens using the relational information (first relational information) at the time of focus detection. PA0 (2) Next, the residual drive amount correction device determines a residual drive amount from the drive amount of the focusing lens, eliminating the movement of the focusing lens after the time of focus detection. PA0 (3) The second conversion device then converts the residual drive amount into a residual defocus amount using the relational information (second relational information) at the time of correction.
The corrected residual defocus amount DF3 is transmitted to the photographic lens side. On the photographic lens side, a residual drive amount Z3' is calculated by multiplying the residual defocus amount DF3 by a conversion coefficient A'. The residual drive amount Z3' is a standard drive value of the focusing lens. The drive mechanism in the photographic lens successively carries out position control or speed control of the focusing lens according to the residual drive amount Z3'.
Furthermore, the coefficients A, A' are values which change moment by moment according to the lens movement. Because of the change of the coefficients A, A', in the photographic lens, data are provided beforehand which indicate the corresponding relationship between the lens position and the conversion coefficient. By referring to the data showing the relationship between lens position and the respective conversion coefficient, a new conversion coefficient can be obtained based on the actual lens position.
However, in the above-described prior art, both the photographic lens side and camera body side acquire the conversion coefficients A, A' independently. Because of the independent acquisition of the conversion coefficients A, A', the conversion coefficient A used on the camera body side and the conversion coefficient A' used on the photographic lens side do not necessarily agree.
Normally, in the case of a whole extension type of photographic lens, the change of the conversion coefficient with lens position is small. Because of the small change of conversion coefficient with the whole extension type photographic lens, the difference between the conversion coefficients A, A' is negligible to a sufficient degree, and focus inaccuracy and the like disadvantages do not arise.
However, in the case of an internal focusing zoom lens or the like, the change of the conversion coefficient with lens position is large. Because of the large change of conversion coefficient when the lens position of the internal focusing zoom lens changes, the conversion coefficients A, A' become greatly displaced between the times of acquisition of the conversion coefficients A, A'.
When the conversion coefficient A and the conversion coefficient A' are greatly displaced in the above-described manner, an inconsistency arises between the actual residual drive amount Z3 found on the camera body side and the residual drive amount Z3' which is actually used in focus control on the photographic lens side. Because of the inconsistency between Z3 and Z3', when using an internal focusing zoom lens or the like, focusing accuracy decreases, focus detection becomes delayed, and the like problems arise.